Many silver alloy compositions are known. Some of these are used for jewelry and flatware, while others are used in brazing compositions and as electrical conductors.
Among these various silver-containing alloys, sterling silver must contain at least 92.5% pure fine silver. The balance of this alloy may be some other metal, but typically includes a substantial percentage of copper. The presence of copper tends to increase the hardness of the resulting alloy.
Sterling silver is typically sold in the form of pellet-like shot. Hence, the user may readily pour out a desired quantity of such shot, melt it, and then cast it into the specific form desired. Investment casting is in widespread use for this purpose. This technique contemplates the formation of a mold into which the molten sterling silver alloy is poured. The material from which the mold is formed, typically contains calcium sulfate.
Silver has a known affinity for oxygen, which affinity increases with temperature. When exposed to air, molten silver will absorb about twenty-two times its volume of oxygen. Like silver, copper also has a great affinity for oxygen, typically forming copper oxide. This may be of the cupric or cuprous variety, or both. Hence, unless air is excluded during the casting process, the cast article may be porous and characterized by the presence of internal voids. Thus, in melting sterling silver and other silver-copper alloys, care must be taken to prevent oxidation.
Copper oxide, also known as fire scale, is typically a darkened portion which blemishes the cast article. Such fire scale is not limited to the surface of the cast article, as in the case of conventional tarnishes, but may penetrate the article to some depth. In some cases, such fire scale may not be removed by buffing and polishing. Moreover, the opportunity for the creation of fire scale exists when the alloy is initially formed as shot, when such shot is melted and recast to form the desired article, and subsequently if the cast article is thereafter annealed. In each of these cases, the alloy is heated, and, given the opportunity, may form fire scale.
As previously noted, the fire scale is more than a surface tarnish. Rather, it is a blemish which may permeate the cast article for some depth, and, in some cases, may not be removed by polishing. To the extent that it exists, the blemish caused by fire scale may lead to the rejection of as-cast parts. Moreover, such rejected parts may have to be re-refined into the elemental metals, and realloyed.
Accordingly, it would be generally desirable to provide an improved silver alloy which substantially reduces the formation of fire scale, which exhibits reduced porosity when recast, and which has a reduced grain size.